Exfoliation corrosion of high strength aluminum alloys occurs when edges of the metal surfaces are exposed to environments containing acids and salts. Aircraft structures, for example, are particularly susceptible to exfoliation corrosion around areas such as fastener holes and other edges, where transverse sections of the microstructure are exposed, effective washing is difficult, and corrosive solutions collect. Exfoliation corrosion produces destructive effects that limit the useful life of aircraft components and other high strength structural aluminum parts.
In the prior art, U.S. Pat. No. 4,092,181 describes a thermomechanical "Method of Imparting a Fine Grain Structure to Aluminum Alloys Having Precipitating Constituents" for creating a fine grain morphology throughout the entire thickness of aluminum alloy sheet material. U.S. Pat. No. 4,799,974 describes a thermomechanical "Method of Forming a Fine Grain Structure on the Surface of an Aluminum Alloy" for creating a fine grain morphology on the entire surface of high strength aluminum alloy sheet material. These methods define the accepted practices for bulk and surface processing of aluminum alloys and teach certain steps that have been deemed necessary to attain a stable fine grain size. The following steps, with only minor variations for expediency or cost considerations, are generally performed in these conventional methods to achieve a fine grain microstructure on the surface of aluminum alloys:
1) Solution treat the material at about 480.degree. C. for 30 minutes to put all second phases into solution; PA1 2) Age the material at about 400.degree. C. for 8 hours to develop a duplex precipitate distribution of both fine and coarse precipitates; PA1 3) Work the surface of the material at moderately low temperatures (rolling at less than about 200.degree. C., for example); PA1 4) Recrystallize the worked material as rapidly as possible (by submersing in a salt bath at about 480.degree. C. for 15 minutes, for example); and PA1 5) Age the material at low temperature for about 24 hours, for example, to achieve appropriate strength levels (such as T-6 and/or T-7, for example).
The foregoing process steps, which are sometimes difficult and lengthy, can add considerably to the cost of producing fine grain aluminum. Conventional through-thickness bulk processing to produce fine grain aluminum is generally limited to sheet material having a thickness less than about 0.08 inch. On the other hand, fine grain surface processing does not provide corrosion protection at locations, such as edges and fastener holes for example, where the microstructure has not been modified. The prior art, as described in U.S. Pat. Nos. 4,092,181 and 4,799,974, does not address the specific need for creating a localized fine grain microstructure along edges and around the openings and interior surfaces of high aspect ratio fastener holes, such as those used in aircraft structures. These locations, however, are the most susceptible sites for initiation of exfoliation corrosion. The prior art process steps listed above, including solution treatment and long time age, are not practical for localized microstructural control nor are they applicable to the particular geometry of fastener holes. In addition, conventional localized surface working procedures (such as shot peening or cold expansion, for example) do not impart uniform or sufficient work for corrosion resistance when applied to aluminum alloy edges and fastener hole surfaces. Shot peening is limited, at best, to low aspect ratio holes (i.e., thin sheets having large diameter holes). Furthermore, shot peening can severely distort the geometry of fastener holes, thus requiring subsequent machining that results in removal of the worked surface. Cold expansion processes, commonly used to impart fatigue resistance to fastener holes, do not effect localized deformation to initiate fine grain recrystallization, and thus do not provide improved corrosion resistance.
In addition to the limitations of prior art fine grain processes, new environmental restrictions prevent the use of coatings previously relied on to impart corrosion resistance to fastener locations in aluminum alloys. Many of the chemicals used in such coating processes are now restricted or banned as harmful to the environment. Thus, there is a need for environmentally acceptable methods for providing corrosion resistance at selected surface locations in aluminum alloy structures such as edges and fastener holes in aircraft components.